In a typical dust/fume collector the dust or fume laden air or gas enters a large housing via an inlet connection. Within the housing is located a tube sheet in which there is positioned a plurality of filter bags, which consist of a filter material stretched over a wire cage. The filter material divides the dust laden air/gas from the clean air/gas which passes through it. In a normal arrangement the tube sheet is horizontal and consequently the filter bags hang vertically below the tube sheet and the dust laden air passes through the filter material. The dust is retained or filtered out on the outside of the filter bags and the cleaned air goes on to pass out the outlet.
The dust level continues to build-up on the outside of the filter bag until the pressure drop across the dust collector becomes too high and the flow rate of the dust laden air decreases. At this stage the filter material of the filter bags must be cleaned. This is done by a flow of a pulse of high pressure air to stop the forward (normal) flow of air/gas and reverse the flow at the filter material surface to remove the accumulated dust/fume.
While this cleaning operation can be done with the filter off-line, that is no forward flow through the filter, it is conventionally carried out whilst the filter is operating continuously.
To clean filter bags during the continuous operating mode, the filter bags are arranged in rows. Above each row a blowtube is mounted to allow all the filter bags in the same row to be cleaned simultaneously. A dust collector may contain anything up to say 20 such rows and during the cleaning operation only one row at a time, is taken out of service, by a reverse pulse cleaning air flow, leaving all other bags operating in the forward filtering mode. During the cleaning operation of a single row of filter bags the cleaning pulse lasts for only a fraction of a second during which time the dust built-up on the outside of the filter material is both shocked by the sudden impulse of cleaning air and blown off the filter surface.
A shortcoming with all types of reverse pulse dust collectors at the present time is that as each row of filter bags is cleaned on-line, only a relatively small amount of the dust that is instantaneously removed from the surface of the filter material directly reaches to the collecting hopper at the bottom of the collector, for subsequent removal. In some cases the amount of dust sent to the hopper can be as low as 10 percent, especially if the air to cloth ratio is high.
The reason for the small amount of dust removal is due to the fact that a lot of the dust is either redeposited back onto the filter surface as soon as the cleaning pulse is stopped, or it is deposited onto the adjacent filter bags. Dust and fume re-deposition occurs after the cleaning pulse ceases, the forward direction airflow resumes immediately, causing any nearby dust/fume particles to redeposit by flowing to the surface of the filter bags in that row. In the case of the adjacent row or rows of bags, the flow continues in the forward filtering mode during the cleaning operation and is always available for deposition.
One particular study, "Performance of a Pulse-Jet filter at high filtration velocity-II Filter Cake Re-deposition." Leith, First & Feldman JAPCA July 1977, showed that when flyash is being filtered at an air to cloth ratio of 10 fpm (feet per minute), the quantity of dust that arrives in the hopper is only 12 percent of the total dust present on each filter bag. Of the remaining 88% of the dust, 38 percent was redeposited on the same bag, whilst 50 percent was deposited on adjacent bags.
One known method of attempting to reduce the re-deposition in a dust collector, is to relocate the dust laden air/gas entry of the dust collector from the normal bottom entry, up to a top entry. In a bottom entry system the dust laden air enters at the top of the hopper where some of the heavy particles drop-out and the remaining air/gas rises up to the filter bags. In the top entry system the dust laden air/gas enters at the top of the filter just below the tube sheet and all air/gas passes downwards past the filter bags. It is this downward component of air/gas flow which helps wash away the dust/fume particles during the cleaning phase.
Whilst this arrangement has some effect on reducing the level of re-deposition, it is only a partial solution and is not accepted by dust collector manufacturers as a good solution to the problem. One reason for poor acceptance by dust collector manufacturers is that the top entry configuration is accompanied by higher capital costs. Another main reason for wanting reduce re-deposition is that existing dust collectors, with bottom entry suffer excessive pressure drop. To change to top entry is very expensive and makes the installation of a new dust collector appear the more attractive alternative.